1. Field of the Invention
The present invention relates to magnetic powder cores and to methods for making the same. In particular, the present invention relates to a low-coercive-force, low-loss magnetic powder core and a method for making the same. The present invention also relates to switching power supplies, various converter circuits, and active filters. Furthermore, the present invention relates to filters and amplifying devices, and particularly, relates to a low-loss filter outputting less distorted waveforms.
2. Description of the Related Art
As magnetic cores used in core components, such as transformer cores for switching power supplies and smoothing choke cores, which require a constant permeability up to the high frequency region, ferrite closed-magnetic-circuit cores, ferrite gapped cores, and amorphous-alloy-tape-wound cores provided with gaps have been proposed. Also, magnetic powder cores formed by compacting a mixture of a powder, such as carbonyl iron, permalloy, or sendust, and an insulating material have been proposed.
Ferrite sintered magnetic cores exhibit low core loss, but simultaneously exhibit small saturation magnetic flux densities. Thus, in ferrite closed-magnetic-circuit cores and ferrite gapped cores, a leakage magnetic flux from the gap section adversely affects peripheral electric circuits. Magnetic powder cores using powders of carbonyl iron, permalloy, and sendust have the disadvantage of large core loss, although the cores exhibit higher saturation magnetic flux densities compared to ferrite magnetic cores.
In recent years, development of electronic devices has advanced with an increase in the use thereof. In particular, the weight of the development was shifted toward reducing heat dissipation by reducing the size of the electronic devices and reducing the power loss. In order to achieve these aims, switching power supplies, various DC/DC converter circuits, and active filters have been improved. These devices use various types of magnetic elements having magnetic cores. Ferrite is mainly used for the magnetic cores. In some cases, carbonyl iron magnetic cores, FeAlSi-alloy magnetic powder cores, and FeNi-alloy magnetic powder cores are also used.
A ferrite magnetic core is generally provided with a gap to prevent magnetic saturation. A leakage magnetic flux from the gap will adversely affect peripheral circuits. On the other hand, a NiZn ferrite core exhibits a large core loss, resulting in high heat dissipation from a device using this core. A carbonyl magnetic powder core exhibits an extremely large core loss, resulting in significantly high heat dissipation compared to ferrite magnetic cores. In addition, in a FeAlSi-alloy magnetic powder core and a FeNi-alloy magnetic powder core, the core loss thereof is lower than that of the carbonyl iron magnetic powder core, but still does not reach required levels.
Low-pass filters have been used for smoothing the pulse shape output from impulse modulation amplifiers. The requirements for low-pass filters are low loss and less distortion of smoothed waveforms. A low-pass filter is generally provided with a capacitor and an inductor composed of a coil with a magnetic core. Achievement of these requirements strongly depends on properties of the magnetic core constituting the inductor. Thus, conventional low-pass filters use amorphous magnetic cores provided with gaps, ferrite cores provided with gaps, or carbonyl iron gap-free magnetic powder cores.
However, in filters using amorphous magnetic cores provided with gaps or ferrite cores provided with gaps, leakage magnetic fields from the gaps may adversely affect peripheral elements and circuits, resulting in decreased stability in the entire circuits including the filters and generation of noise. Moreover, in these filters, the amplitude permeability varies with changes in the magnetic field and exhibits a large rate of change. When a pulsed current causing a large change in magnetic field is smoothed, the waveform will be significantly distorted.
In the carbonyl iron gap-free magnetic powder cores, the dependence of the amplitude permeability on the magnetic field is constant, and the waveform is not distorted. However, the carbonyl iron gap-free magnetic powder cores dissipate a significant amount of heat due to large core loss.
The large core loss in conventional magnetic powder cores is due to large core loss of the magnetic materials themselves used for the magnetic powder and insufficient relaxation of stress which is applied during compacting of the magnetic powder cores.
Accordingly, it is an object of the present invention to provide a magnetic powder core having low coercive force and low core loss and a method for making the same.
It is another object of the present invention to provide a switching power supply, converter circuits, and active filters which exhibit low heat dissipation and which can be miniaturized.
It is another object of the present invention to provide a filter which dissipates less heat due to low loss and which suppresses waveform distortion, and an amplifying device provided with this filter.
According to a first aspect of the present invention, a magnetic powder core comprises a molded article of a mixture of a glassy alloy powder and an insulating material, the glassy alloy comprising Fe and at least one element selected from Al, P, C, Si, and B, having a texture primarily composed of an amorphous phase, and exhibiting a temperature difference xcex94Tx, which is represented by the equation xcex94Tx=Txxe2x88x92Tg, of at least 20 K in a supercooled liquid, wherein Tx indicates the crystallization temperature and Tg indicates the glass transition temperature.
Since the magnetic powder core of the present invention comprises a mixture of the glassy alloy powder and the insulating material, the insulating material enhances the resistivity of the entire magnetic powder core. Thus, the magnetic powder core exhibits reduced core loss due to reduced eddy current loss and high permeability in a high-frequency region.
Preferably, the glassy alloy has a resistivity of at least 1.5 xcexcxcexa9xe2x80xa2m. The eddy current loss in the glassy alloy particles in a high-frequency region is thereby effectively decreased, the magnetic powder core exhibiting further reduced core loss.
The magnetic powder has a coercive force of preferably 80 A/m or less and more preferably 40 A/m or less in an applied magnetic field of xc2x12.4 kA/m.
Preferably, the magnetic powder core has a core loss of 400 kW/m3 or less under the conditions of a frequency of 100 kHz and a magnetic flux density of 0.1 T. This core loss is significantly smaller than that of known magnetic powder cores.
Preferably, the insulating material comprises a silicone rubber. The silicone rubber is effective for relieving the internal stress of the magnetic powder core.
Preferably, the glassy alloy is represented by the following formula:
(Fe1-aTa)100-x-v-z-wAlx(P1-bSib)vCzBw 
wherein T represents at least one element of Co and Ni, and the subscripts a, b, x, v, z, and w satisfy the relationships, 0xe2x89xa6axe2x89xa60.15 by atomic ratio, 0 less than bxe2x89xa60.8 by atomic ratio, 0 atomic percent less than xxe2x89xa620 atomic percent, 0 atomic percent less than vxe2x89xa622 atomic percent, 0 atomic percent less than zxe2x89xa612 atomic percent, and 0 atomic percent less than wxe2x89xa616 atomic percent.
The magnetic powder core of the present invention is formed of the above Fe-based glassy alloy powder in which the Fe content is higher than the Co and/or Ni content. Since this Fe-based glassy alloy exhibits higher saturation magnetic flux density than that of a Co-based glassy alloy, the magnetic powder core exhibits further improved magnetic characteristics.
According to a second aspect of the present invention, a method for making a magnetic powder core comprises a powder preparation step of preparing a powder of a glassy alloy comprising Fe and at least one element selected from Al, P, C, Si, and B, having a texture primarily composed of an amorphous phase, and exhibiting a temperature difference xcex94Tx, which is represented by the equation xcex94Tx=Txxe2x88x92Tg, of at least 20 K in a supercooled liquid, wherein Tx indicates the crystallization temperature and Tg indicates the glass transition temperature, a molding step of mixing the glassy alloy powder with an insulating material and compacting the mixture to form a magnetic core precursor, and an annealing step of annealing the magnetic core precursor at a temperature in the range between (Tgxe2x88x92170) K and Tg K to relieve the internal stress of the magnetic core precursor.
Preferably, the magnetic core precursor is annealed at a temperature between (Tgxe2x88x92140) K and (Tgxe2x88x9260) K in the annealing step. The internal stress formed in the glassy alloy or the magnetic core precursor during the powder preparation step or the molding step is relieved without crystallization of the glassy alloy.
More preferably, the magnetic core precursor is annealed at a temperature between (Tgxe2x88x92140) K and (Tgxe2x88x9260) K. When the magnetic core precursor is annealed at a temperature in the above range, the resulting magnetic powder core exhibits a coercive force of 80 A/m or less in an applied magnetic field of xc2x12.4 kA/m.
More preferably, the magnetic core precursor is annealed at a temperature between (Tgxe2x88x92110) K and (Tgxe2x88x9260) K. When the magnetic core precursor is annealed at a temperature in the above range, the resulting magnetic powder core exhibits a coercive force of 40 A/m or less in an applied magnetic field of xc2x12.4 kA/m.
In this method, the glassy alloy is preferably represented by the following formula:
(Fe1-aTa)100-x-v-z-wAlx(P1-bSib)vCzBw 
wherein T represents at least one element of Co and Ni, and the subscripts a, b, x, v, z, and w satisfy the relationships, 0xe2x89xa6axe2x89xa60.15 by atomic ratio, 0 less than bxe2x89xa60.8 by atomic ratio, 0 atomic percent less than xxe2x89xa620 atomic percent, 0 atomic percent less than vxe2x89xa622 atomic percent, 0 atomic percent less than zxe2x89xa612 atomic percent, and 0 atomic percent less than wxe2x89xa616 atomic percent.
According to a third aspect of the present invention, a switching power supply comprises a switching element for converting a DC voltage into a rectangular waveform voltage, a transformer for transforming the rectangular waveform voltage, and a rectification circuit and a smoothing circuit for converting the transformed rectangular waveform voltage into a DC voltage, wherein the transformer comprises a magnetic core comprising a molded article of a mixture of a glassy alloy powder and an insulating material, the glassy alloy powder having a texture primarily composed of an amorphous phase and exhibiting a temperature difference xcex94Tx, which is represented by the equation xcex94Tx=Txxe2x88x92Tg, of at least 20 K in a supercooled liquid, wherein Tx indicates the crystallization temperature and Tg indicates the glass transition temperature.
Since the switching power supply of the present invention includes a transformer having a magnetic core composed of a glassy alloy powder and an insulating material, the internal stress of the magnetic core can be relieved by annealing at a temperature which is sufficiently lower than the crystallization temperature of the glassy alloy, and the heat dissipation from the entire switching power supply can be reduced due to reduced core loss.
The magnetic core exhibiting low permeability does not require a gap for preventing magnetic saturation, and does not generate a leakage magnetic field which adversely affects other peripheral circuit.
According to a fourth aspect of the present invention, a switching power supply comprises a switching element for converting a DC voltage into a rectangular waveform voltage, a transformer for transforming the rectangular waveform voltage, and a rectification circuit and a smoothing circuit for converting the transformed rectangular waveform voltage into a DC voltage, wherein the smoothing circuit comprises a capacitor and a coil provided with a magnetic core, the magnetic core comprising a molded article of a mixture of a glassy alloy powder and an insulating material, the glassy alloy powder comprising Fe and at least one element selected from Al, P, C, Si, and B, having a texture primarily composed of an amorphous phase, and exhibiting a temperature difference xcex94Tx, which is represented-by the equation xcex94Tx=Txxe2x88x92Tg, of at least 20 K in a supercooled liquid, wherein Tx indicates the crystallization temperature and Tg indicates the glass transition temperature.
Since the switching power supply of the present invention includes a transformer having a magnetic core composed of a glassy alloy powder, the internal stress of the magnetic core can be relieved by annealing at a temperature which is sufficiently lower than the crystallization temperature of the glassy alloy, and the heat dissipation from the entire switching power supply can be reduced due to reduced core loss.
The magnetic core exhibiting low permeability does not require a gap for preventing magnetic saturation, and does not generate a leakage magnetic field which adversely affects other peripheral circuit.
According to a fifth aspect of the present invention, a step-down converter circuit comprises a switching element, a coil provided with a magnetic core generating a back electromotive force when the switching element breaks a DC current, a capacitor for smoothing a current generated by the back electromotive force, and a rectifying element connected to the coil provided with the magnetic core in an antiparallel state, the rectifying element, the coil provided with the magnetic core, and the capacitor constituting a circulating current path, wherein the magnetic core comprises a molded article of a mixture of a glassy alloy powder and an insulating material, the glassy alloy having a texture primarily composed of an amorphous phase and exhibiting a temperature difference xcex94Tx, which is represented by the equation xcex94Tx=Txxe2x88x92Tg, of at least 20 K in a supercooled liquid, wherein Tx indicates the crystallization temperature and Tg indicates the glass transition temperature.
According to a sixth aspect of the present invention, a boosting converter circuit comprises a switching element, a coil provided with a magnetic core generating a back electromotive force when the switching element breaks a DC current, a rectifying element connected in series in the forward direction to the coil provided with the magnetic core for rectifying a current generated by the back electromotive force, and a capacitor for smoothing the rectified current, wherein the magnetic core comprises a molded article of a mixture of a glassy alloy powder and an insulating material, the glassy alloy having a texture primarily composed of an amorphous phase and exhibiting a temperature difference xcex94Tx, which is represented by the equation xcex94Tx=Txxe2x88x92Tg, of at least 20 K in a supercooled liquid, wherein Tx indicates the crystallization temperature and Tg indicates the glass transition temperature.
According to a seventh aspect of the present invention, a polarity-reversing converter circuit comprises a switching element, a coil provided with a magnetic core generating a back electromotive force when the switching element breaks a DC current, a capacitor for smoothing a current generated by the back electromotive force, and a rectifying element connected in series in the backward direction to the coil provided with the magnetic core for blocking the DC current, wherein the magnetic core comprises a molded article of a mixture of a glassy alloy powder and an insulating material, the glassy alloy having a texture primarily composed of an amorphous phase and exhibiting a temperature difference xcex94Tx which is represented by the equation xcex94Tx=Txxe2x88x92Tg, of at least 20 K in a supercooled liquid, wherein Tx indicates the crystallization temperature and Tg indicates the glass transition temperature.
In the step-down converter circuit, the boosting converter circuit, and the polarity-reversing converter circuit, a magnetic core composed of a glassy alloy powder is used. Thus, the internal stress of the magnetic core can be relieved by annealing at a temperature which is sufficiently lower than the crystallization temperature of the glassy alloy, and the heat dissipation from the entire switching power supply can be reduced due to reduced core loss.
The magnetic core exhibiting low permeability does not require a gap for preventing magnetic saturation, and does not generate a leakage magnetic field which adversely affects other peripheral circuit.
According to an eighth aspect of the present invention, an active filter comprises the above-described boosting converter circuit, and a control unit for controlling the switching interval of the switching element of the boosting converter circuit.
The active filter of the present invention uses a coil with a magnetic core composed of a glassy alloy powder in the converter circuit therein. Since this magnetic core exhibits low loss, the heat dissipation from the entire active filter can be reduced.
The magnetic core exhibiting low permeability does not require a gap for preventing magnetic saturation, and does not generate a leakage magnetic field which adversely affects other peripheral circuits.
In the above aspects, the magnetic core exhibits low core loss and low permeability, reducing heat dissipation. Moreover, the magnetic core exhibiting low permeability does not require a gap for preventing magnetic saturation, and does not generate a leakage magnetic field, which adversely affects other peripheral circuit.
Moreover, the insulating material enhances the resistivity of the entire magnetic core and further reduces core loss due to reduced eddy current loss.
According to a ninth aspect of the present invention, a filter comprises a capacitor and an inductor of a coil wound around a magnetic core, wherein the magnetic core comprises a molded article of a mixture of a glassy alloy powder and an insulating material, the glassy alloy having a texture primarily composed of an amorphous phase and exhibiting a temperature difference xcex94Tx, which is represented by the equation xcex94Tx=Txxe2x88x92Tg, of at least 20 K in a supercooled liquid, wherein Tx indicates the crystallization temperature and Tg indicates the glass transition temperature.
In this filter, the internal stress of the glassy alloy can be relieved by annealing at a temperature which is sufficiently lower than the crystallization temperature of the glassy alloy, and the magnetic core exhibits low core loss and a substantially constant amplitude permeability over a wide intensity range of magnetic field. Thus, the filter exhibits reduced heat dissipation and outputs less distorted waveforms.
Moreover, the insulating material enhances the resistivity of the entire magnetic core and further reduces core loss due to reduced eddy current loss. Since high permeability is maintained in a high-frequency region, the filter exhibits further improved high-frequency characteristics.
Preferably, the rate of change in amplitude permeability of the magnetic core in a magnetic field of 2,000 A/m is within xc2x110% of an amplitude permeability in a magnetic field of 200 A/m, and the permeability of the magnetic core at 100 kHz is in the range of 50 to 200.
The filter outputs less distorted waveforms. Thus, the filter is preferably applicable to a smoothing circuit of a pulse width modulating amplifier.
Preferably, the filter is a low-pass filter. That is, the capacitor and the inductor are connected into an L shape.
Preferably, the glassy alloy is represented by the following formula:
(Fe1-a2Ta2)100-x2-v2-z2-w2Alx2(P1-b2Sib2)v2Cz2Bw2 
wherein T represents at least one element of Co and Ni, and the subscripts a2, b2, x2, v2, z2, and w2 satisfy the relationships, 0xe2x89xa6a2xe2x89xa60.15 by atomic ratio, 0xe2x89xa6b2xe2x89xa60.8 by atomic ratio, 0 atomic percent less than x2xe2x89xa620 atomic percent, 0 atomic percent less than v2xe2x89xa622 atomic percent, 0 atomic percent less than z2xe2x89xa612 atomic percent, and 0 atomic percent less than w2xe2x89xa616 atomic percent.
Since the magnetic core composed of the glassy alloy having the above composition exhibits reduced core loss and a substantially constant amplitude permeability over a variable magnetic field, the filter using the magnetic core exhibits reduced loss and reduced heat dissipation, and outputs waveforms with less distortion.
According to a tenth aspect of the present invention, an amplifying device comprises an amplifier for outputting a pulsed current and a filter connected to the output side of the amplifier for smoothing the pulsed current, wherein the filter comprises a capacitor and an inductor of a coil wound around a magnetic core, wherein the magnetic core comprises a molded article of a mixture of a glassy alloy powder and an insulating material, the glassy alloy having a texture primarily composed of an amorphous phase and exhibiting a temperature difference xcex94Tx, which is represented by the equation xcex94Tx=Txxe2x88x92Tg, of at least 20 K in a supercooled liquid, wherein Tx indicates the crystallization temperature and Tg indicates the glass transition temperature.
In the amplifying device of the present invention, the magnetic core composed of the glassy alloy powder and the insulating material. The internal stress of the magnetic core can be relieved by annealing at a temperature which is sufficiently lower than the crystallization temperature of the glassy alloy, and the heat dissipation from the amplifying device can be reduced due to reduced core loss. The amplifying device outputs waveforms with less distortion.
Moreover, the insulating material enhances the resistivity of the entire magnetic core and further reduces core loss due to reduced eddy current loss. Since high permeability is maintained in a high-frequency region, the filter exhibits reduced loss and outputs waveforms with less distortion.
Preferably, the rate of change in amplitude permeability of the magnetic core in a magnetic field of 2,000 A/m is within xc2x110% of an amplitude permeability in a magnetic field of 200 A/m, and the permeability of the magnetic core at 100 kHz is in the range of 50 to 200.
Within the above rate of change, the output waveform from the amplifying device is less distorted. Moreover, the number of turns of the coil can be reduced, thus resulting in a reduction in size of the amplifying device.
Preferably, the filter is a low-pass filter.
Preferably, the amplifier is a pulse-width-modulation amplifier.
preferably, the glassy alloy is represented by the following formula:
(Fe1-a2Ta2)100-x2-v2-z2-w2Alx2(P1-b2Sib2)v2Cz2Bw2 
wherein T represents at least one element of Co and Ni, and the subscripts a2, b2, x2, v2, z2, and w2 satisfy the relationships, 0xe2x89xa6a2xe2x89xa60.15 by atomic ratio, 0xe2x89xa6b2xe2x89xa60.8 by atomic ratio, 0 atomic percent less than x2xe2x89xa620 atomic percent, 0 atomic percent less than v2xe2x89xa622 atomic percent, 0 atomic percent less than z2xe2x89xa612 atomic percent, and 0 atomic percent less than w2xe2x89xa616 atomic percent.
Since the magnetic core composed of the glassy alloy having the above composition exhibits reduced core loss and a substantially constant amplitude permeability over a variable magnetic field, the amplifying device using the magnetic core exhibits reduced loss and outputs waveforms with less distortion.